There is an optical digital coherent receiver that carries out coherent reception of optical signals and executes digital processing of the received signals. Furthermore, there is a configuration in which an automatic level control (ALC) processing unit that adjusts the signal level of an output of an adaptive equalization unit to a target value is set in an optical digital coherent receiver. This ALC processing unit may generate a histogram obtained by counting the number of samples regarding discrete monitored values corresponding to the amplitude value of the output from the adaptive equalization unit and decide a level adjustment coefficient by which the output of the adaptive equalization unit is to be multiplied in such a manner that the monitored value of the peak value of the histogram becomes the target value.
However, as described above, there is a problem that the adjustment error of the amplitude becomes large and the error rate becomes high in a multi-valued modulation system such as the 16 quadrature amplitude modulation (QAM).
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 2012-114599.